Image coding method and image decoding method based on edge direction

ABSTRACT

Provided is an image coding method for performing intra prediction achieving higher coding efficiency. The method for coding image data on a block-by-block basis includes: generating a predicted block by predicting a current block; computing a difference between the current block and the predicted block; coding the difference computed in the computing; decoding the difference coded in the coding; and adding the difference decoded in the decoding to the predicted block to generate a decoded block, wherein the generating includes: detecting an edge in a previously decoded block corresponding to a block adjacent to the current block; and extrapolating or interpolating previously decoded image data corresponding to a pixel included in the block adjacent to the current block, along a direction of the edge detected in the detecting to generate the predicted block.

TECHNICAL FIELD

The present invention relates to an image coding method forcompression-coding image data and video data with improved codingefficiency, an image decoding method for reconstructing the image dataand video data that have been compression-coded, and a correspondingimage coding apparatus and image decoding apparatus.

BACKGROUND ART

An ever increasing number of applications for video-conferencing,digital video broadcasting, and streaming of video contents over theInternet such as for video-on-demand services relies on a transmissionof video information. When video data is being transmitted or recorded,a substantial amount of data has to be sent through conventionaltransmission channels of limited bandwidth or has to be stored onconventional storage media of limited data capacity. In order totransmit video information through conventional channels and store it onstorage media, it is inevitable to compress or reduce the volume ofdigital data.

For the compression of video data, a plurality of video coding standardshas been developed. Such video standards are, for instance, ITU-Tstandards denoted with H.26× and ISO/IEC standards denoted with MPEG-x.The most up-to-date and advanced video coding standard is currently thestandard denoted as H.264/MPEG-4 AVC (see Non-Patent Reference 1).

The coding approach underlying most of these standards is based onpredictive coding including the following main stages (a) to (d):

(a) dividing video frames into blocks of pixels in order to subject eachvideo frame to data compression on a block-by-block basis;

(b) identifying temporal and spatial redundancies by predicting theindividual blocks from previously coded video data;

(c) removing the identified redundancies by subtracting the predicteddata from the video data; and

(d) compressing the remaining data by means of Fourier transformation,quantization, and entropy coding.

Current video coding standards differ in the prediction modes that areemployed for predicting each macroblock. Most video coding standards usemotion estimation and motion compensation to predict video data frompreviously coded and decoded frames (inter-frame prediction).Alternatively, block data may also be extrapolated from neighboringblocks of the same frame (intra-frame prediction). The H.264/AVCstandard defines several intra-frame prediction modes that differ, forexample, with respect to the reference pixels used for an extrapolationor with respect to the direction in which the pixels are extrapolated.

FIGS. 1A and 1B illustrate intra-prediction modes according to theH.264/AVC standard. As illustrated in FIG. 1A, a current block 10 of 4×4pixels is predicted by extrapolating a set of thirteen reference pixels20 located on and to the left of the current block 10 to be predicted.With this prediction, a predicted block corresponding to the currentblock 10 is generated. In order to perform the extrapolation, one out of8 possible extrapolation directions (intra-prediction directions) shownin FIG. 1B may be selected. In addition, a DC-mode which employs a meanvalue of the reference pixels 20 for predicting the current block 10 maybe selected.

In intra-frame prediction, spatial prediction on subblocks of sizes of4×4, 8×8, or 16×16 pixels is performed in order to reduce the spatialredundancies existing in natural images. Pixel values of already codedsurrounding blocks (pixel values of reference pixels) are used topredict the pixel values of the current block. These reference pixelsare extrapolated according to predefined prediction modes. In H.264/AVC,there are 9 prediction modes for subblocks of 4×4 and 8×8 pixels, and 4prediction modes for macroblocks of 16×16 pixels.

For subblocks smaller than 8×8 pixels, the 9 prediction modes include 8prediction directions (cf. FIG. 1B), suitable for predicting directionalstructures, and a DC prediction mode. In the DC prediction mode, allpixels of the current block are predicted with a single value, which isthe mean value of the surrounding reference pixels. In the directionalmodes, the reference pixels are repeated along the correspondingdirection. According to the vertical mode, for example, reference pixelsin a row immediately above the current block are vertically repeated(extrapolated). According to the horizontal mode, on the other hand,pixels in a column immediately to the left of the current block arehorizontally repeated (extrapolated). The remaining modes are diagonalprediction modes derived in similar ways.

-   Non-Patent Reference 1: ISO/IEC 14496-10 “MPEG-4 Part 10 Advanced    Video Coding”

DISCLOSURE OF INVENTION Problems that Invention is to Solve

However, when the current block includes a sharp linear edge and anappropriate predicted block cannot be generated in the conventionaltechnique, there is a problem that an amount of coded data increases ordistortion occurs in a coded image. The problem will be morespecifically described hereinafter.

These conventional prediction directions are limited. In other words,the intra-prediction directions are limited to 8 directions asillustrated in FIG. 1B. Thus, intra prediction in 1 out of 8 directionscan only be performed in the conventional technique.

If there is a sharp linear edge, an intra-prediction mode having anangle closest to the edge is selected from among the 8 directionsillustrated in FIG. 1B, and the current block will be predicted by theselected prediction mode. However, when an angle of an intra-predictionmode is slightly different from the actual angle of the edge, thepredicted block will include an edge along a direction different from adirection of the actual edge. Thus, a differential image computed bysubtracting the predicted block from the current block has highfrequency components that are the furthest from the reference pixels.

Since the high frequency components normally need many bits for coding,the amount of coded data increases. Alternatively, when the highfrequency components are eliminated for suppressing the increased amountof coded data, distortion occurs in a coded image.

The object of the present invention is to solve the problem and providean improved method and apparatus for coding and decoding image data andvideo data so that a higher coding efficiency can be achieved and thecoded distortion can be reduced.

Means to Solve the Problems

In order to solve the problem, the image coding method according to anaspect of the present invention is a method for coding image data on ablock-by-block basis, and the method includes: partitioning the imagedata into a plurality of blocks; generating a predicted block bypredicting a current block that is one of the plurality of blocks;computing a difference between the current block and the predictedblock; coding the difference computed in the computing; decoding thedifference coded in the coding; and adding the difference decoded in thedecoding to the predicted block to generate a decoded block, wherein thegenerating includes: detecting an edge in a previously decoded blockcorresponding to a block adjacent to the current block; andextrapolating or interpolating previously decoded image data along adirection of the edge detected in the detecting to generate thepredicted block, the previously decoded image data corresponding to apixel included in the block adjacent to the current block.

Thereby, a predicted block can be generated using a direction of theedge detected from the surrounding blocks of the current block, as aprediction direction. Thus, when the current block includes a sharpedge, a more suitable predicted block than the predicted block generatedusing the conventional predetermined directional prediction modes can begenerated. Since the prediction residual that is a difference betweenthe current block and the predicted block contains less information, thecoded distortion can be reduced and higher coding efficiency can beachieved.

Furthermore, the previously decoded image data may be linearlyextrapolated or interpolated in the extrapolating or interpolating togenerate the predicted block.

Furthermore, a weighted sum of at least 2 pixel values of a plurality ofdecoded pixels included in the previously decoded image data may becomputed for each predicted pixel included in the predicted block, thecomputed weighted sum may be extrapolated or interpolated to generatethe predicted block in the extrapolating or interpolating, and weightsfor computing the weighted sum may be determined according to thedirection of the edge detected in the detecting.

Thereby, a suitable predicted value can be computed by computation of aweighted sum of adjacent pixels even when there is no integer pixel thatcrosses a predicted pixel subject to the computation of the predictedvalue and is on an extension of the detected edge.

Furthermore, each of the weights may be determined according to theposition of the predicted pixel. More specifically, a larger value maybe assigned to each of the weights, as a distance from one of theplurality of decoded pixels to be used for generating the predictedblock, to one of the predicted pixels is smaller.

Furthermore, at least 2 edges having directions different from eachother may be detected in the detecting, the pixel values of theplurality of decoded pixels for extrapolation or interpolation along thedirections of the at least 2 edges may be combined for each of thepredicted pixels, and the combined pixel values may be extrapolated orinterpolated to generate the predicted block in the extrapolating orinterpolating.

Thereby, even when edges are detected from the surrounding blocks of thecurrent block, a suitable predicted value can be computed by computationof predicted values along the respective directions of the edges.

Furthermore, the weighted sum may be computed for each of the predictedpixels by multiplying each of the weights by a corresponding one of thepixel values of the plurality of decoded pixels, the computed weightedsum may be extrapolated or interpolated to generate the predicted blockin the extrapolating or interpolating, and a larger value may beassigned to each of the weights, as a distance from one of the pluralityof decoded pixels to the previously decoded block in which one of the atleast 2 edges is detected is smaller, the plurality of decoded pixelsrespectively corresponding to the weights.

Furthermore, a larger value may be assigned to each of the weights, as adistance from one of the plurality of decoded pixels to an extension ofone of the at least 2 edges is smaller, the plurality of decoded pixelsrespectively corresponding to the weights.

Furthermore, the generating may further include selecting one of aplurality of prediction modes including an edge prediction mode, andwhen the edge prediction mode is selected, the previously decoded imagedata may be extrapolated or interpolated along the direction of the edgedetected in the detecting to generate the predicted block in theextrapolating or interpolating.

Thereby, when the edge is detected, the edge prediction mode can beimplemented in which data is extrapolated or interpolated along adirection of the detected edge. When no edge is detected, a standardprediction mode can be implemented.

Furthermore, the plurality of prediction modes may further include a DCprediction mode and a plurality of directional prediction modes thathave been predefined, and in the extrapolating or interpolating, pixelvalues of the previously decoded image data may be averaged to generatethe predicted block when the DC prediction mode is selected, and thepreviously decoded image data may be extrapolated along a directioncorresponding to a selected one of the plurality of directionalprediction modes to generate the predicted block, when the one of theplurality of directional prediction modes is selected.

Furthermore, in the selecting: (i) one of the edge prediction modes andthe plurality of directional prediction modes may be selected when theedge is detected in the detecting; and (ii) one of the DC predictionmode and the plurality of directional prediction modes may be selectedwhen the edge is not detected in the detecting.

Thereby, the optimal prediction modes, namely, the edge prediction modeand the conventional directional prediction modes can be selected ascandidates for the prediction modes to be used when the edge isdetected, and the optimal prediction modes, namely, the DC predictionmode and the conventional directional prediction modes can be selectedas candidates for the prediction modes to be used when the edge is notdetected. Here, when an edge is detected, it is unlikely that thecurrent block is flat, and usually, a difference between the predictedblock generated in the DC prediction mode and the current block containslots of information. Thus, the coding efficiency with the DC predictionmode is low, and the DC prediction mode is hardly used when the edge isdetected. Accordingly, the DC prediction mode can be eliminated from acandidate for the prediction modes.

Furthermore, in the coding: (i) when one of the plurality of directionalprediction modes is selected in the selecting, a mode indicatorindicating the selected one of plurality of directional prediction modesmay be coded; and (ii) when one of the DC prediction mode and the edgeprediction mode is selected, a mode indicator indicating the selectedone of the DC prediction mode and the edge prediction mode may be coded.

Since information indicating the conventional DC prediction mode can beused as information indicating the edge prediction mode, informationindicating the edge prediction mode does not have to be newly coded, andthus higher coding efficiency can be achieved. As described above, it isunlikely that the DC prediction mode is selected in the case where theedge prediction mode can be selected, such as a case where an edge isdetected. Furthermore, it is unlikely that the edge prediction mode isselected when the DC prediction mode can be selected. Since there ishardly a case where both the edge prediction mode and the DC predictionmode are necessary as candidates for the prediction modes, informationindicating the DC prediction mode can be assigned as informationindicating the edge prediction mode.

Furthermore, in the selecting: one of the edge prediction mode and theplurality of directional prediction modes may be selected when the edgeis detected in the detecting and the direction of the edge detected inthe detecting points to the current block; and one of the DC predictionmode and the plurality of directional prediction modes may be selectedwhen (i) the edge is not detected in the detecting, or (ii) the edge isdetected in the detecting and the direction of the edge detected in thedetecting does not point to the current block.

Furthermore, the detecting may include: computing a gradient vector foreach of a plurality of pixels included in the previously generated(coded and decoded) decoded block corresponding to the block adjacent tothe current block; determining whether at least one of the gradientvectors computed in the computing satisfies conditions that (i) a normof the at least one of the gradient vectors exceeds a predeterminedthreshold and (ii) a vector having a direction perpendicular to adirection of the at least one of the gradient vectors points to thecurrent block; and detecting an edge having the direction perpendicularto the direction of at least one of the gradient vectors when the atleast one of the gradient vectors computed in the computing satisfiesthe conditions in the determining.

Since the edge can be detected using not the current block but blockssurrounding the current block, there is no need to transmit, to adecoder, information necessary for the edge prediction mode, such asinformation indicating a direction of the edge. Thus, higher codingefficiency can be achieved.

Furthermore, in the detecting of the edge having the direction, thedirection of the edge may be determined based on a direction of agradient vector having a largest norm, the gradient vector correspondingto one of the at least one of the gradient vectors satisfying theconditions.

Furthermore, in the detecting of the edge having the direction, thedirection of the edge may be determined based on a direction obtained byaveraging directions of the at least one of the gradient vectorssatisfying the conditions.

Furthermore, the image decoding method according to an aspect of thepresent invention is a method for decoding image data including a codedprediction residual on a block-by-block basis, and the method includes:decoding the prediction residual of a current block; generating apredicted block by predicting the current block; and adding thepredicted block generated in the generating to the prediction residualdecoded in the decoding to generate a decoded block, wherein thegenerating includes: detecting an edge in a previously decoded blockcorresponding to a block adjacent to the current block; andextrapolating or interpolating previously decoded image data along adirection of the edge detected in the detecting to generate thepredicted block, the previously decoded image data corresponding to apixel included in the block adjacent to the current block.

Thereby, a predicted block can be generated using a direction of theedge detected from the surrounding blocks of the current block, as aprediction direction. Thus, when the current block includes a sharpedge, a more suitable predicted block than the predicted block generatedusing the conventional predetermined directional prediction modes can begenerated. Since the prediction residual that is a difference betweenthe current block and the predicted block contains less information, thecoded distortion can be reduced.

Furthermore, a weighted sum of at least 2 pixel values of a plurality ofdecoded pixels included in the previously decoded image data may becomputed for each predicted pixel included in the predicted block, thecomputed weighted sum may be extrapolated or interpolated to generatethe predicted block in the extrapolating or interpolating, and weightsfor computing the weighted sum may be determined according to thedirection of the edge detected in the detecting.

Thereby, a suitable predicted value can be computed by computation of aweighted sum of adjacent pixels even when there is no integer pixel thatcrosses a predicted pixel subject to the computation of the predictedvalue and is on an extension of the detected edge.

Furthermore, at least 2 edges having directions different from eachother may be detected in the detecting, the pixel values of theplurality of decoded pixels for extrapolation or interpolation along thedirections of the at least 2 edges may be combined for each of thepredicted pixels, and the combined pixel values may be extrapolated orinterpolated to generate the predicted block in the extrapolating orinterpolating.

Thereby, even when edges are detected from the surrounding blocks of thecurrent block, a suitable predicted value can be computed by computationof predicted values along the respective directions of the edges.

Furthermore, the generating may further include selecting one of aplurality of prediction modes including an edge prediction mode, andwhen the edge prediction mode is selected, the previously decoded imagedata may be extrapolated or interpolated along the direction of the edgedetected in the detecting to generate the predicted block in theextrapolating or interpolating.

Thereby, when the edge is detected, the edge prediction mode can beimplemented in which data is extrapolated or interpolated along adirection of the detected edge. When the edge is not detected, thestandard prediction mode can be implemented.

Furthermore, in the selecting: (i) one of the edge prediction mode andthe plurality of directional prediction modes may be selected when theedge is detected in the detecting; and (ii) one of the DC predictionmode and the plurality of directional prediction modes may be selectedwhen the edge is not detected in the detecting.

Thereby, the optimal prediction modes, namely, the edge prediction modeand the conventional directional prediction modes can be selected ascandidates for the prediction modes to be used when the edge isdetected, and the optimal prediction modes, namely, the DC predictionmode and the conventional directional prediction modes can be selectedas candidates for the prediction modes to be used when the edge is notdetected.

Furthermore, the image data may further include a prediction modeindicator indicating a prediction mode, and (ii) in the selecting, theedge prediction mode may be selected when the prediction mode indicatorindicates one of the DC prediction mode and the edge prediction mode andthe edge is detected in the detecting, and the DC prediction mode may beselected when the edge is not detected in the detecting.

Since a bitstream coded using information indicating the conventional DCprediction mode as information indicating the edge prediction mode canbe accurately decoded, the information indicating the edge predictionmode does not have to be newly coded, and higher coding efficiency canbe achieved.

Furthermore, the detecting may include: computing a gradient vector foreach of a plurality of pixels included in the previously decoded blockcorresponding to the block adjacent to the current block; determiningwhether at least one of the gradient vectors computed in the computingsatisfies conditions that (i) a norm of the at least one of the gradientvectors exceeds a predetermined threshold and (ii) a vector having adirection perpendicular to a direction of the at least one of thegradient vectors points to the current block; and detecting an edgehaving the direction perpendicular to the direction of at least one ofthe gradient vectors when the at least one of the gradient vectorscomputed in the computing satisfies the conditions in the determining.

Since the edge can be detected using not the current block but blockssurrounding the current block, the bitstream can be accurately decodedwithout transmission, from an encoder, information necessary for theedge prediction mode, such as information indicating a direction of theedge.

An aspect of the present invention is not only implemented as an imagecoding method and an image decoding method but also as an image codingapparatus and an image decoding apparatus using respective stepsincluded in the image coding method and image decoding method asprocessing units. Furthermore, these steps may be implemented as aprogram causing a computer to execute such steps. Furthermore, anotheraspect of the present invention can be implemented as a recording mediumthat records the program, such as a computer-readable CD-ROM, and asinformation, data, or a signal that indicates the program. Furthermore,these program, information, data, and signal may be distributed througha communication network, such as the Internet.

A part or all of the constituent elements included in the respectiveimage coding apparatus and image decoding apparatus may be configuredfrom a single System Large-Scale Integration (LSI). The System LSI is asuper-multi-function LSI manufactured by integrating constituent unitson one chip, and is specifically a computer system configured byincluding a microprocessor, a ROM, a RAM, and so on.

Effects of the Invention

According to an aspect of the present invention, the current block canbe accurately predicted. Thus, the coded distortion can be reduced andhigher coding efficiency can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates an example of a relationship between referencepixels and the current block in which intra prediction is employedaccording to the conventional H.264/AVC standard.

FIG. 1B illustrates prediction directions included in theintra-prediction mode set according to the conventional H.264/AVCstandard.

FIG. 2 is a block diagram illustrating an example of a configuration ofan image coding apparatus with hybrid coding scheme of an embodiment.

FIG. 3 illustrates detailed configuration of the intra-prediction modedetermination unit included in the image coding apparatus according tothe embodiment.

FIG. 4 is a schematic drawing illustrating a method for detecting anedge and estimating a direction of the detected edge.

FIG. 5 is a block diagram illustrating an example of a configuration ofthe image decoding apparatus according to the embodiment.

FIG. 6 illustrates an example of detailed configuration of anintra-prediction unit included in the image decoding apparatus accordingto the embodiment.

FIG. 7 is a flowchart illustrating edge detection according to theembodiment.

FIG. 8 is a flowchart illustrating a process for determining aprediction mode set according to the embodiment.

FIG. 9 is a flowchart illustrating a process for coding image data orvideo data based on intra prediction according to the embodiment.

FIG. 10 is a flowchart illustrating a process for computing an angle ofan edge according to the embodiment.

FIG. 11A schematically illustrates a process for generating a predictedblock according to the edge prediction mode by extrapolating image datathat has been previously coded and decoded according to the embodiment.

FIG. 11B schematically illustrates a process for generating a predictedblock according to the edge prediction mode by extrapolating image datathat has been previously coded and decoded according to the embodiment.

FIG. 12 schematically illustrates edge prediction by interpolating imagedata that has been previously coded and decoded according to theembodiment.

FIG. 13 is a flowchart illustrating a method for generating a predictedblock according to the edge prediction mode in the embodiment.

FIG. 14 schematically illustrates a process for generating a predictedblock according to the edge prediction mode using at least 2 edges inthe embodiment.

FIG. 15 is a flowchart illustrating a process for decoding image dataand video data using intra prediction according to the embodiment.

FIG. 16 is a flowchart illustrating a method for intra predictionaccording to the embodiment.

FIG. 17A shows a rate distortion curve for the first 10 frames of thesequence Foreman (CIF).

FIG. 17B shows a rate distortion curve for the sequence SpinCalendar(720p).

FIG. 18 is a chart indicating the percentage of blocks coded using theedge prediction mode and the bitrate reduction percentage.

FIG. 19 is a block diagram illustrating an example of a configuration ofan image coding apparatus having a configuration different from that ofthe image coding apparatus of the embodiment.

FIG. 20 is a schematic drawing illustrating an example of pixels to bestored in a memory according to the embodiment.

NUMERICAL REFERENCES

-   10 Current block-   20 Reference pixel-   30 Surrounding block-   40, 41, 61, 62 Edge-   50, 51, 52, 65, 66 Edge direction-   63, 64 Current pixel-   71, 72 Pixel group-   100, 500 Image coding apparatus-   101, 304 Frame memory-   102 Subtracting unit-   103 Frequency transform unit-   104 Quantization unit-   105, 505 Variable length coding unit-   106, 301 Inverse quantization unit-   107, 302 Inverse frequency transform unit-   108, 303 Adding unit-   109, 509 Reference picture memory-   110, 510 Intra-prediction mode determination unit-   111 Motion estimation unit-   112, 305 Intra-prediction unit-   113, 306 Motion compensation unit-   114, 115, 308, 309 Switch-   116, 307 Control unit-   117 Probability table holding unit-   118 Coding mode determination unit-   120 Differential coding unit-   130 Differential decoding unit-   201, 401 Edge detection unit-   202, 402 Direction determination unit-   203, 403 Norm determination unit-   204 Prediction mode set determination unit-   205 Prediction mode selection unit-   300 Image decoding apparatus-   404 Prediction mode determination unit-   405 Predicted block generation unit-   519 Threshold determination unit

BEST MODE FOR CARRYING OUT THE INVENTION

The image coding apparatus and image decoding apparatus according to anembodiment can detect an edge included in surrounding blocks of thecurrent block, and allow the use of an edge prediction mode in which adirection of the detected edge is used as an intra-prediction direction.In other words, the embodiment makes it possible to performintra-prediction coding on a picture including a sharp edge by takingthe direction of the detected edge into accounts.

FIG. 2 is a block diagram illustrating an example of a configuration ofan image coding apparatus 100 for hybrid video coding according to theembodiment. The image coding apparatus 100 includes a frame memory 101,a subtracting unit 102, a frequency transform unit 103, a quantizationunit 104, a variable length coding unit 105, an inverse quantizationunit 106, an inverse frequency transform unit 107, an adding unit 108, areference picture memory 109, an intra-prediction mode determinationunit 110, a motion estimation unit 111, an intra-prediction unit 112, amotion compensation unit 113, switches 114 and 115, a control unit 116,a probability table holding unit 117, and a coding mode determinationunit 118. As illustrated in FIG. 2, the frequency transform unit 103 andthe quantization unit 104 are included in a differential coding unit120. Furthermore, the inverse quantization unit 106 and the inversefrequency transform unit 107 are included in a differential decodingunit 130.

Processes for each processing unit will be described along withoperations when the image coding apparatus 100 codes input video datamade up of frames.

Each picture of the input video data is stored in the frame memory 101,divided into blocks, and then outputted from the frame memory 101 on ablock-by-block basis (e.g. in units of macroblocks having horizontal 16pixels and vertical 16 pixels). Here, the input video data may be in aprogressive mode and an interlaced mode.

Each macroblock can be coded either in intra- or in inter-predictionmode. First, a case where the current block is coded in intra-predictionmode will be described.

In case of an intra-prediction mode (intra-frame prediction), themacroblock outputted from the frame memory 101 is inputted to theintra-prediction mode determination unit 110 (a switch 114 is connectedto a terminal “a” by the control unit 116). The intra-prediction modedetermination unit 110 determines how to perform intra prediction on theinputted macroblock. More specifically, as an intra-prediction mode(IPM), it is necessary to determine an intra predicted block size (oneof the following sizes: horizontal 4 pixels×vertical 4 pixels;horizontal 8 pixels×vertical 8 pixels; and horizontal 16 pixels×vertical16 pixels), and an intra-prediction direction. The detailedconfiguration of the intra-prediction mode determination unit 110 willbe described later with reference to FIG. 3.

A current block 10 (horizontal 4 pixels×vertical 4 pixels) in FIG. 1Amay for instance be predicted according to 8 predefined intra-predictiondirections using reference pixels 20. Here, it is assumed that thereference pixels 20 (diagonally hatched squares in FIG. 1A) to be usedfor the intra prediction have already been coded and decoded, and storedin the reference picture memory 109. Information indicating thedetermined intra-prediction mode IPM is outputted to theintra-prediction unit 112 and the coding mode determination unit 118.

The intra-prediction unit 112, based on the intra-prediction mode IPMdetermined by the intra-prediction mode determination unit 110, obtainsreference pixels to be used for intra prediction (intra referencepixels) from the reference picture memory 109, generates anintra-predicted image IP by extrapolating or interpolating pixel valuesof the reference pixels, and outputs the generated intra-predicted imageIP to the subtracting unit 102 (a switch 115 is connected to a terminal“a” by the control unit 116).

The subtracting unit 102 receives (i) a macroblock (current macroblock)of an input picture included in the input video data from the framememory 101 and (ii) the intra-predicted image IP generated by theintra-prediction unit 112, generates a differential image by computing adifference (also referred to as prediction residual) between the currentmacroblock and the intra-predicted image IP, and outputs thedifferential image to the frequency transform unit 103.

The frequency transform unit 103 performs a frequency transformation,such as a discrete cosine transformation, on the differential imagegenerated by the subtracting unit 102, generates frequency transformcoefficients, and outputs the generated frequency transformcoefficients.

The quantization unit 104 quantizes the frequency transform coefficientsgenerated by the frequency transform unit 103, and outputs quantizedfrequency transform coefficients QT. Here, the quantization is a processfor dividing the frequency transform coefficients by a predeterminedvalue (quantization step). It is assumed that this quantization step isgiven by the control unit 116 (the quantization step may be included ina control signal CTL to be inputted to the control unit 116). Thequantized frequency transform coefficients QT are outputted to thevariable length coding unit 105 and the inverse quantization unit 106.

The inverse quantization unit 106 inverse-quantizes the quantizedfrequency transform coefficients QT, and outputs the inverse-quantizedfrequency transform coefficients to the inverse frequency transform unit107. Here, a quantization step identical to the quantization step usedfor the quantization by the quantization unit 104 is fed from thecontrol unit 116 to the inverse quantization unit 106.

The inverse frequency transform unit 107 generates a decodeddifferential image LDD by inverse-frequency transforming theinverse-quantized frequency transform coefficients. The inversefrequency transform unit 107 outputs the generated decoded differentialimage LDD to the adding unit 108.

The adding unit 108 generates a decoded image LD by adding the decodeddifferential image LDD to one of the predicted image IP and a predictedimage MP, and stores the decoded image LD in the reference picturememory 109. The decoded image LD stored in the reference picture memory109 is used in a later coding as a reference picture.

The variable length coding unit 105 performs variable length coding on(i) the quantized frequency transform coefficients QT inputted from thequantization unit 104 and (ii) information indicating theintra-prediction mode IPM inputted from the intra-prediction modedetermination unit 110 through the coding mode determination unit 118,and outputs a bitstream which is also referred to as a code sequence.

Here, among the variable length coding methods used in the variablelength coding unit 105, there is a context adaptive arithmetic codingmethod used in the international standard H.264 for coding movingpictures. The context adaptive arithmetic coding method is a method forswitching probability tables used for arithmetic coding according tovariable length coding target data and data on which the variable lengthcoding has already been performed (context adaptation). For example, ablock size for intra prediction, a block size for frequencytransformation, and the like are used as a context for performingvariable length coding on the quantized frequency transform coefficientsQT. Here, it is assumed that the probability tables are held in theprobability table holding unit 117.

Next, a case where the current macroblock is coded in inter-predictionmode will be described.

In case of inter-prediction mode (inter-frame prediction), themacroblock outputted from the frame memory 101 is inputted to the motionestimation unit 111 (the switch 114 is connected to a terminal “b” bythe control unit 116). The motion estimation unit 111 estimates a motion(motion vector) of the inputted macroblock with respect to the referencepicture (a reconstructed picture that is held in the reference picturememory 109 and is different from the picture to be coded). In the motionestimation, generally the following motion vector is selected: a motionvector having a minimum differential value between the block to be codedand a predicted image, and having a minimum sum of weights for a codedamount of the motion vector. The estimated motion vector is outputted tothe motion compensation unit 113 and the coding mode determination unit118.

The motion compensation unit 113 obtains reference pixels used in interprediction (inter reference pixels) from the reference picture memory109, based on the motion vector determined by the motion estimation unit111. Then, the motion compensation unit 113 generates a predicted imageMP, and outputs the predicted image MP to the subtracting unit 102 (theswitch 115 is connected to a terminal “b” by the control unit 116).

The processes performed by the subtracting unit 102, the frequencytransform unit 103, the quantization unit 104, the inverse quantizationunit 106, the inverse frequency transform unit 107, and the adding unit108 are the same as the processes described in the case of intraprediction. Thus, the explanation about the processes is omitted here.

The coding mode determination unit 118 determines a coding mode of amacroblock to be coded, using outputs from the intra-prediction modedetermination unit 110, motion estimation unit 111, quantization unit104, frame memory 101, and adding unit 108, and the like. Here, it isdetermined which one of the intra-picture prediction coding and theinter-picture prediction coding is used for coding the macroblock to becoded. The coding mode determination unit 118 determines, in general, acoding mode having the smallest value of a sum of weights between anamount of bits to be generated and coded distortion. In the case wherethe intra-picture prediction coding is selected as the coding mode, thecoding mode determination unit 118 outputs a coding mode MD andinformation indicating the intra-prediction mode IPM (such as a blocksize for intra prediction and an intra-prediction direction) to thevariable length coding unit 105. In the case where the inter-pictureprediction coding is selected as a coding mode, the coding modedetermination unit 118 outputs a motion vector MV and the coding mode MDto the variable length coding unit 105.

For example, a cost function using a bitrate and coded distortion thatare of the standard H.264 may be used to determine the best predictionmode for coding the current block. The differential image isorthogonally transformed, quantized, and variable-length coded for eachprediction mode. Then, a bitrate and resulting coded distortion for eachprediction mode are computed. For example, the Lagrangian cost functionJ is used as a cost function.J=D+λ·R  [Equation 1]

In Equation 1, R denotes a bitrate used to code a differential image(also referred to as prediction residual) and prediction modeinformation. D denotes the coded distortion, and λ denotes theLagrangian multiplier computed according to a quantization parameter QPchosen for the coding. The prediction mode with the lowest cost functionJ is chosen to predict the current block.

Here, the coding mode determination unit 118 includes a memory fortemporarily storing the cost function J to determine the best predictionmode.

The variable length coding unit 105 performs variable length coding onquantized frequency transform coefficients QT inputted from thequantization unit 104, and the coding mode MD and the intra-predictionmode IPM or the motion vector MV that are outputted from the coding modedetermination unit 118, and outputs a bitstream.

Here, in the case where the variable length coding unit 105 codes themotion vector MV using the context adaptive arithmetic coding method, amethod for changing a probability table depending on each size (context)of the motion vectors of coded surrounding blocks can be used. Here,probability tables are held in the probability table holding unit 117.

A prediction mode contains a full set of information that is required bya decoder (for example, an image decoding apparatus 300 to be describedlater and see FIG. 5) to reproduce the prediction performed by anencoder (image coding apparatus 100) in the process for coding videodata. Therefore, a prediction mode defines the coding mode for eachmacroblock, i.e., whether intra or inter prediction is applied. Further,the prediction mode includes information on how the macroblock issubdivided. According to H.264/AVC, a macroblock made up of 16×16 pixelsmay, for instance in case of intra prediction, be further subdividedinto blocks of 8×8 or 4×4 pixels each.

Depending on a coding mode, the prediction mode further includesinformation for specifying (i) a set of motion vectors employed formotion compensation or (ii) the intra-prediction mode applied tointra-predict the current block.

Next, a detailed configuration of the intra-prediction modedetermination unit 110 according to the embodiment will be describedwith reference to FIG. 3.

FIG. 3 is a block diagram illustrating a detailed configuration of theintra-prediction mode determination unit 110. As illustrated, theintra-prediction mode determination unit 110 includes an edge detectionunit 201, a direction determination unit 202, a norm determination unit203, a prediction mode set determination unit 204, and a prediction modeselection unit 205.

The edge detection unit 201 reads a reference image from the referencepicture memory 109, and detects an edge included in the read referenceimage. For example, the edge detection unit 201 computes a gradientvector field of the reference image, evaluates a norm of gradients whichreach a maximum value of an edge. A direction of the edge is obtainedfrom a vector that is perpendicular to a direction of a correspondinggradient.

A reference image for the edge detection is made up of pixels includedin a block that has already been coded and decoded, out of pixelsbelonging to blocks directly adjacent to the current block. Gradientvalues are computed in the neighboring pixels using the vertical andhorizontal Sobel operators as expressed in the following Equation 2.

$\begin{matrix}{{{Sobel}_{x} = \begin{bmatrix}{- 1} & 0 & 1 \\{- 2} & 0 & 2 \\{- 1} & 0 & 1\end{bmatrix}},{{Sobel}_{y} = \begin{bmatrix}{- 1} & {- 2} & {- 1} \\0 & 0 & 0 \\1 & 2 & 1\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The edge detection unit 201 computes a magnitude of a gradient (gradientvalue) of a corresponding pixel in a horizontal direction and a verticaldirection for each pixel, using the vertical and horizontal Sobeloperators. The direction of the corresponding gradient is determined foreach pixel by combining vectors of gradient values in the computedhorizontal direction and vertical direction.

FIG. 4 is a schematic drawing illustrating a method for detecting anedge and estimating a direction of the detected edge. A regionleft-diagonally hatched represents pixels of surrounding blocks 30 thatare considered for computing gradients. The edge detection unit 201computes a gradient value for each pixel left-diagonally hatched in FIG.4, using Equation 2, computes a direction of a gradient using thecomputed gradient value, and determines a direction perpendicular to thecomputed direction as a direction of the edge. A vector of the directionperpendicular to the direction of the gradient is also referred to as anedge vector. Here, a norm of the edge vector is, for example, the sameas a norm of a gradient vector of a pixel having the edge vectordetected.

With reference to FIG. 3, the direction determination unit 202determines whether or not the direction of the edge detected by the edgedetection unit 201 points to the current block for intra prediction.When an edge 40 in FIG. 4 is detected, for example, an extension of theedge 40 crosses pixels belonging to the current block 10. Thus, thedirection determination unit 202 determines that the edge 40 points tothe current block 10 for intra prediction. In contrast, when an edge 41in FIG. 4 is detected, an extension of the edge 41 does not cross anypixel belonging to the current block 10. Thus, the directiondetermination unit 202 determines that the edge 41 does not point to thecurrent block 10 for intra prediction.

The norm determination unit 203 computes a norm of an edge determined bythe direction determination unit 202 as pointing to the current blockfor intra prediction, and determines whether or not the computed norm islarger than a predetermined threshold. The norm of an edge is a norm ofan edge vector or a gradient vector. The norm determination unit 203determines that the edge is detected when the computed norm is largerthan the predetermined threshold. Furthermore, the norm determinationunit 203 determines that the edge is not detected when the computed normis not more than the predetermined threshold. The threshold used hereinis empirically selected, for example, by comparing differences betweendifferent norms of maximum vectors and between different sequencesincluding both sharp edges and smooth areas.

The prediction mode set determination unit 204 determines a predictionmode set for use in coding the current block, depending on whether ornot an edge is detected. More specifically, the prediction mode setdetermination unit 204 determines either a prediction mode set includingthe conventional DC prediction mode and the 8 prediction direction modes(a conventional prediction mode set) or a prediction mode set includingthe edge prediction mode and the 8 prediction direction modes (a newprediction mode set).

When a norm of an edge vector is not more than the predeterminedthreshold, the prediction mode set determination unit 204 determines theconventional prediction mode set including the 9 prediction modes foruse in H.264/AVC video coding standard, as an available prediction modeset.

When a norm of an edge vector (or gradient vector) is larger than thepredetermined threshold, the edge is detected. Thus, the prediction modeset determination unit 204 determines the new prediction mode set as anavailable prediction mode set. When the edge is detected, the DCprediction mode is probably not a good candidate for predicting thecurrent block, because the block is likely to contain a strongdirectional structure. Accordingly, the DC prediction mode is replacedwith the edge prediction mode. The remaining 8 directional predictionmodes of the H.264 video coding standard, however, are held. Suchprediction modes are especially useful when a change in the edgedirection occurs between the previous block (block from which an edge isdetected) and the current block.

The prediction mode selection unit 205 selects a prediction mode out ofthe prediction mode set determined by the prediction mode setdetermination unit 204.

As described above, the image coding apparatus 100 of the embodimentincludes the new edge prediction mode as a prediction mode to beselected when the current block is coded. Furthermore, the image codingapparatus 100 detects an edge from blocks surrounding the current blockfor intra-coding the current block according to the edge predictionmode, and performs intra-prediction coding using a direction of thedetected edge as an intra-prediction direction. The specific operationsof the image coding apparatus 100 of the embodiment will be describedwith reference to a flowchart later.

Next, a configuration of the image decoding apparatus 300 will bedescribed.

FIG. 5 is a block diagram illustrating an example of the configurationof the image decoding apparatus 300 according to the embodiment. Theimage decoding apparatus 300 includes an inverse quantization unit 301,an inverse frequency transform unit 302, an adding unit 303, a framememory 304, an intra-prediction unit 305, a motion compensation unit306, a control unit 307, and switches 308 and 309.

The processes for each processing unit when the image decoding apparatus300 decodes a bitstream (coded video data) will be describedhereinafter. The image decoding apparatus 300 decodes the coded videodata including a prediction residual for each block either as an intra-or inter-frame prediction coded block. The coding mode MD is inputted tothe control unit 307, information indicating the intra-prediction modeIPM or the motion vector MV is inputted to the switch 308, and thequantized frequency transform coefficients QT are inputted to theinverse quantization unit 301.

The control unit 307 controls the switches 308 and 309 based on thecoding mode MD. In the case where the coding mode MD indicates theintra-prediction coding, the switch 308 is connected to the terminal“a”, and the information indicating the intra-prediction mode IPM isinputted to the intra-prediction unit 305. In the case where the codingmode MD indicates the inter-picture prediction coding, the switch 308 isconnected to the terminal “b”, and the motion vector MV is inputted tothe motion compensation unit 306.

In the case where the current block is an intra-prediction coded block,the switches 308 and 309 are connected to the terminal “a”. Theinformation indicating the intra-prediction mode is thus inputted to theIntra-prediction unit 305 and the quantized frequency transformcoefficients QT are inputted to the inverse quantization unit 301. Here,the quantized frequency transform coefficients QT correspond to theprediction residual coded by an encoder (for example, image codingapparatus 100).

The intra-prediction unit 305, based on the inputted intra-predictionmode, obtains intra-predicted reference pixels from the frame memory304, generates an intra-predicted image (predicted block), and outputsthe intra-predicted image to the adding unit 303.

The inverse quantization unit 301 inverse-quantizes the quantizedfrequency transform coefficients QT, and outputs the inverse-quantizedfrequency transform coefficients to the inverse frequency transform unit302. The inverse-quantized frequency transform coefficients are furtherinverse-frequency transformed by the inverse frequency transform unit302, and become a decoded differential image LDD. Then, the inversefrequency transform unit 302 outputs the decoded differential image LDDto the adding unit 303.

The decoded differential image LDD is added to the intra-predicted imageIP by the adding unit 303 so as to generate a decoded image LD, and thedecoded image LD is stored in the frame memory 304. The decoded image LDstored in the frame memory 304 is used as a reference picture for laterdecoding. In addition, it is outputted to form the decoded video data.

In the case where the current block is an inter predicted block, theswitches 308 and 309 are connected to the terminal “b”. The informationindicating the motion vector MV is inputted to the motion compensationunit 306, and the quantized frequency transform coefficients QT areinputted to the inverse quantization unit 301.

The motion compensation unit 306, based on the inputted motion vectorMV, obtains reference pixels from the frame memory 304, generates apredicted picture, and outputs the predicted picture to the adding unit303.

The processes of the inverse quantization unit 301, inverse frequencytransform unit 302, and adding unit 303 are the same as the processesexplained in the case of intra predicted blocks. The decoded images LDsare stored in the frame memory 304. The decoded images stored in theframe memory 304 are used as reference pictures for later decoding. Inaddition, the decoded images are outputted to form the decoded videodata.

Next, the detailed configuration of the intra-prediction unit 305according to the embodiment will be described with reference to FIG. 6.

FIG. 6 is a block diagram illustrating an example of the detailedconfiguration of the intra-prediction unit 305 according to theembodiment. As illustrated in FIG. 6, the intra-prediction unit 305includes an edge detection unit 401, a direction determination unit 402,a norm determination unit 403, a prediction mode determination unit 404,and a predicted block generation unit 405.

The edge detection unit 401 reads a reference image from the framememory 304 when the prediction mode determination unit 404 determinesone of the edge prediction mode and the DC prediction mode as theintra-prediction mode, and detects an edge included in the readreference image. The details of the edge detection are the same as thoseof the edge detection unit 201, and they will be described later.

The direction determination unit 402 determines whether or not thedirection of the edge detected by the edge detection unit 401 points tothe current block for intra prediction. The details of the edgedirection determination processes are the same as those of the directiondetermination unit 202, and they will be described later.

The norm determination unit 403 computes a norm of an edge determined bythe direction determination unit 402 as indicating the current block forintra prediction, and determines whether or not the computed norm islarger than a predetermined threshold. The predetermined threshold ispreferably the same threshold for use in the norm determination processin coding the current block. Thus, when the threshold is not a valueunique to the image decoding apparatus 300, the image coding apparatus100 notifies the predetermined threshold. The details of the normdetermination process are the same as those of the norm determinationunit 203, and they will be described later.

The prediction mode determination unit 404 obtains informationindicating the intra-prediction mode IPM included in the bitstream, anddetermines whether or not the intra-prediction mode IPM is one of theedge prediction mode and the DC prediction mode. The prediction modedetermination unit 404 causes the edge detection unit 401 to detect anedge when the intra-prediction mode IPM is one of the edge predictionmode and the DC prediction mode. Then, the prediction mode determinationunit 404 notifies the predicted block generation unit 405 whether theintra-prediction mode IPM is one of the edge prediction mode and the DCprediction mode, according to a result of the determination by the normdetermination unit 403.

When the intra-prediction mode IPM is neither the edge prediction modenor the DC prediction mode, in other words, when the intra-predictionmode IPM is one of the conventional directional prediction modes (FIG.1B), the prediction mode determination unit 404 notifies the predictedblock generation unit 405 that the intra-prediction mode IPM is thedirectional prediction mode.

The predicted block generation unit 405 generates a predicted blockaccording to the intra-prediction mode IPM notified by the predictionmode determination unit 404. More specifically, the predicted blockgeneration unit 405 reads reference pixels from the frame memory 304,and generates a predicted block by extrapolating or interpolating theread reference pixels. The generated predicted block is outputted to theadding unit 303 through the switch 309.

As described above, the image decoding apparatus 300 of the embodimentincludes the new edge prediction mode as a prediction mode to beselected when the current block is decoded. Furthermore, the imagedecoding apparatus 300 detects an edge from blocks surrounding thecurrent block for decoding the current block according to the edgeprediction mode, and performs intra-prediction coding using a directionof the detected edge as an intra-prediction direction. The specificoperations of the image decoding apparatus 300 of the embodiment will bedescribed with reference to a flowchart later.

Next, the operations of the image coding apparatus 100 according to theembodiment will be described. First, edge detection will be describedwith reference to FIG. 7. FIG. 7 is a flowchart illustrating edgedetection according to the embodiment.

The edge detection is performed on all pixels that are considered forcomputing gradients (pixels left-diagonally hatched in FIG. 4) (S101).

The edge detection unit 201 computes a gradient vector at a currentpixel position by using the Sobel operators of Equation 2 (S102). Theedge detection unit 201 computes an edge vector having a directionperpendicular to a direction of the computed gradient vector and detectsthe direction of the computed edge vector (S103).

Next, the direction determination unit 202 determines whether thegradient represents an edge that crosses the current block, i.e.,whether the edge vector having the direction perpendicular to thedirection of the computed gradient vector points to the current block tobe predicted (S104). When the edge vector does not point to the currentblock to be predicted (No in S104) and there is a next pixel (Yes inS105), the process proceeds to the next pixel. Specifically, computationof a gradient vector (S102) is repeated on the next pixel.

When the edge vector points to the current block to be predicted (Yes inS104), the norm determination unit 203 computes a norm of the gradientvector (S106). Then, the norm determination unit 203 determines whetheror not the computed norm of the gradient vector is larger than apredetermined threshold (S107). When the computed norm is not more thanthe predetermined threshold (No in S107) and there is a next pixel (Yesin S105), the process proceeds to the next pixel. Specifically,computation of a gradient vector (S102) is repeated on the next pixel.

When the norm of the gradient vector is larger than the norm of thepredetermined threshold (Yes in S107), the norm determination unit 203determines whether or not the computed norm is larger than a norm of thegradient vector having the largest norm of all previously determinedgradient vectors (S108). When the norm is larger than the largest norm(Yes in S108), the vector is marked as the new maximum vector (S109).

The norm determination unit 203 stores the vector and the position ofthe pixel in a memory, independent of a result of the comparison betweenthe computed norm and the largest norm (S110). Here, the vector storedin the memory is preferably an edge vector. Furthermore, the memory isincluded in the norm determination unit 203, for example.

When there is a next pixel (Yes in S105), the process proceeds to thenext pixel. Specifically, computation of a gradient vector (S102) isrepeated on the next pixel.

As described above, the edge detection is performed on all of the pixelsthat are included in the surrounding blocks 30 surrounding the currentblock (current block 10) and that are subject to the edge detection(pixels left-diagonally hatched in FIG. 4), and the position of thepixels from which edges are detected and each of the edge vectors arestored in the memory.

According to the embodiment of the present invention, anintra-prediction mode set that may be employed for coding a block ofimage data depends on the result of the edge detection performed onadjacent, previously coded and decoded blocks. In other words, an edgeis detected not from the current block but from surrounding blocks, andthe intra-prediction mode set for coding the current block is determinedbased on a result of the detection.

The same holds true for the image decoding apparatus 300 when performingthe edge detection. More specifically, the processes performed by theedge detection unit 201, the direction determination unit 202, and thenorm determination unit 203 are respectively performed by the edgedetection unit 401, the direction determination unit 402, and the normdetermination unit 403.

Next, a process for determining an intra-prediction mode set will bedescribed with reference to FIG. 8. FIG. 8 is a flowchart illustratingthe process for determining a suitable intra-prediction mode set.

First, the edge detection in FIG. 7 is performed by the edge detectionunit 201, the direction determination unit 202, and the normdetermination unit 203 (S201). Next, the prediction mode setdetermination unit 204 determines whether or not at least one vector isstored, i.e., whether or not there is a vector with a norm larger than athreshold along a direction of an edge that points to the current block.

When at least one vector is stored, i.e., an edge is detected (Yes inS202), the prediction mode set determination unit 204 selects the newprediction mode set as an available intra-prediction mode set (S203).The new edge prediction mode set includes the 8 conventional predictiondirections (the directional prediction modes of the H.264 codingstandard) and the new edge prediction mode.

When no edge vector is stored, i.e., an edge is not detected (No inS202), the prediction mode set determination unit 204 selects theconventional prediction mode set as the available intra-prediction modeset (S204). As described above, the conventional prediction mode setincludes the 9 conventional prediction modes of the H.264 codingstandard, i.e., the 8 conventional prediction directions and theconventional DC prediction mode.

As described above, the image coding apparatus 100 of the embodimentdetermines an intra-prediction mode set depending on whether or not anedge is detected. More specifically, the image coding apparatus 100selects the intra-prediction mode set including the edge prediction modewhen an edge is detected, while selecting the intra-prediction mode setincluding the DC prediction mode when an edge is not detected.

In the subsequent processes, the prediction mode selection unit 205selects an intra-prediction mode out of the available prediction modeset for performing intra prediction using the intra-prediction unit 112.

Next, a coding process based on intra prediction by the image codingapparatus 100 according to the embodiment will be described withreference to FIG. 9. FIG. 9 is a flowchart illustrating a process forcoding image data or video data based on intra prediction according tothe embodiment.

The coding process in FIG. 9 is performed on each macroblock of 16×16pixels (S301). Furthermore, the coding process is performed on eachpossible partition size obtained by partitioning a macroblock intoblocks (4×4, 8×8, 16×16 pixels) (S302). Furthermore, the coding processis performed on each block obtained by partitioning the partition size(4×4 pixels) (S303).

The intra-prediction mode determination unit 110 executes the processesdescribed with reference to FIG. 8, and selects a suitableintra-prediction mode set (S304). The optimum prediction mode is thenselected by performing the processes (Steps 305 to 314) for eachprediction mode included in the intra-prediction mode set.

More specifically, the prediction mode selection unit 205 selects one ofthe 9 prediction modes included in the intra-prediction mode set, as acurrent prediction mode for a current block B (S305). The selectedcurrent prediction mode is outputted to the intra-prediction unit 112.

The intra-prediction unit 112 generates a predicted block B_(pred)corresponding to the current block B according to the current predictionmode (S306). The generated predicted block B_(pred) is outputted to thesubtracting unit 102.

The subtracting unit 102 computes a difference between the current blockB and the predicted block B_(pred) i.e., a prediction residual(prediction error) B_(res)=B−B_(pred) (S307). The computed predictionresidual B_(res) is outputted to the frequency transform unit 103.

The frequency transform unit 103 performs frequency transformation onthe computed prediction residual B_(res), and outputs thefrequency-transformed prediction residual (frequency transformcoefficients) to the quantization unit 104. The quantization unit 104quantizes the frequency-transformed prediction residual (S308). Thequantized frequency transform coefficients QT generated through thequantization are outputted to the inverse quantization unit 106 and thevariable length coding unit 105.

The inverse quantization unit 106 inverse-quantizes the quantizedfrequency transform coefficients QT, and outputs the inverse-quantizedfrequency transform coefficients to the inverse frequency transform unit107. The inverse frequency transform unit 107 generates a decodeddifference block B_(resq) by inverse-frequency transforming theinverse-quantized frequency transform coefficients (S309). The generateddifference block B_(resq) is outputted to the adding unit 108. Theadding unit 108 adds the difference block B_(resq) to the predictedblock B_(pred) to generate the decoded block B_(pred)+B_(resq).

The coding mode determination unit 118 computes (i) a coded distortionD=B−(B_(pred)+B_(resq)) which is a difference between the current blockB that is the original block and a reconstructed blockB_(pred)+B_(resq), and (ii) a bitrate R=R_(res)+R_(mode) which is a sumof the bitrate required for coding the prediction residual B_(res) andthe current prediction mode (S310). Furthermore, the coding modedetermination unit 118 computes a cost function J=D+λR, using thecomputed coded distortion D and the bitrate R (S311).

The coding mode determination unit 118 determines whether or not thecurrent value of the cost function J is smaller than a minimum valueJmin that has been previously stored (S312). When the computed costfunction J is smaller than the minimum value Jmin (Yes in S312), thecoding mode determination unit 118 stores the current prediction modeand the current partition size in a memory, and updates the previouslystored minimum value Jmin to the computed current value of the costfunction J (S313).

Next, when there is a next prediction mode, i.e., there is a predictionmode in which the cost function J has not yet been computed, among theprediction modes included in the determined prediction mode set (Yes inS314), the prediction mode selection unit 205 selects the predictionmode in which the cost function J has not yet been computed. Then, theprocessing is repeated from the process for generating a predicted block(S306), using the selected prediction mode as the current predictionmode.

When the cost function J is computed for all prediction modes (No inS314) and there is a next block (Yes in S315), the processing isrepeated from the process for determining a prediction mode set (S304),using the next block as the current block. The presence of the nextblock indicates that there is a block in which the cost function J hasnot yet been computed, among all blocks included in one macroblock.

When there is no next block (No in S315) but there is a next partitionsize (Yes in S316), a macroblock is partitioned by the next partitionsize, and the following processing is repeated for each block accordingto the partition. The presence of the next partition size indicates thatthere is a partition size in which processing for each block has not yetbeen performed, among partition sizes obtainable by partitioning onemacroblock. For example, since one macroblock can be partitioned into 3partition sizes (4×4, 8×8, 16×16 pixels), the processes are repeated oneach block partitioned into 3 partition sizes (S302 to S316).

Once all partitions have been processed (No in S316), the coding modedetermination unit 118 selects a partition size with the lowest value ofthe cost function and the corresponding prediction modes (S317).Information indicating the selected partition size and the predictionmodes is outputted to the variable length coding unit 105.

Finally, the variable length coding unit 105 codes the selectedpartition size, prediction modes, and the corresponding residuals, andwrites a result of the coding to an output bitstream (S318). Here, thevariable length coding unit 105 performs the coding, for instance, bymeans of a statistical coder.

Here, information indicating prediction modes in coding (mode indicator)is: (i) information indicating directional prediction modes when thedirectional prediction modes are selected, and (ii) informationindicating that, when one of the DC prediction mode and the edgeprediction mode is selected, the selected prediction mode is one of theDC prediction mode and the edge prediction mode. More specifically, thevariable length coding unit 105 allocates one code word to both the DCprediction mode and the edge prediction mode.

Then, when there is a next macroblock (Yes in S319), the processes (S301to S319) are then repeated on the next macroblock.

As described above, the image coding apparatus 100 of the embodimentdetermines a prediction mode for each block, and codes the input videodata. Furthermore, a prediction mode set is determined before aprediction mode is selected. In the embodiment, candidate predictionmode sets include the conventional prediction mode set and the newprediction mode set. The new prediction mode set includes the edgeprediction mode instead of the DC prediction mode.

Here, the information indicating the edge prediction mode that is a newprediction mode is coded using the code word identical to the code wordto be used for the information indicating the DC prediction mode. Thus,a decoder that receives the code word determines whether the informationindicates the DC prediction mode or the edge prediction mode. Sincethere is no need to allocate a new code word to information indicatingthe edge prediction mode, increase in the amount of coded data can beprevented.

Hereinafter, the new edge prediction mode will be explained in furtherdetail.

As explained above, the gradients of the surrounding pixels are computedfor each pixel subject to the computation. The intra-prediction unit 112computes an angle of an edge, using a vector having the maximum norm andvectors in pixels adjacent to a pixel corresponding to the vector havingthe maximum norm. More specifically, in each of the adjacent pixels, anorm of a corresponding edge vector (or gradient vector) is comparedwith a norm of the maximum vector. The edge vectors of the adjacentpixels are stored in a memory only when the norm of the edge vectors islarger than half the norm of the maximum vector, so that the edgevectors of the adjacent pixels can be used. Then, the intra-predictionunit 112 computes an average of angles between the maximum vector andeach of the surrounding vectors that are stored in a memory, as an angleα of a direction of the new edge prediction mode. More specifically, theprocess is performed according to a flowchart in FIG. 10.

FIG. 10 is a flowchart illustrating a process for computing an angle ofan edge according to the embodiment. The process depends on a result ofthe edge detection described with reference to FIG. 7. As the result ofthe edge detection in FIG. 7, positions of pixels corresponding to 1 ormore edge vectors that are determined to be larger than a threshold arestored in a memory.

The norm determination unit 203 computes an angle of the maximum edgevector detected in the edge detection, and stores the angle in thememory (S401). Then, the following processes are performed on all pixelsthat are adjacent to a pixel at which the maximum edge vector has beencomputed (S402). In other words, the norm determination unit 203performs the following processes using a position of one of the pixelsadjacent to the pixel at which the maximum edge vector has beencomputed, as the current pixel position.

Next, the norm determination unit 203 determines whether an edge vectorcorresponding to the current pixel position is stored in the memoryincluded in the norm determination unit 203 (S403). In other words, thenorm determination unit 203 determines whether an edge vector has a normlarger than a threshold and the edge vector pointing to the currentblock is stored in the memory.

When the edge vector is stored (Yes in S403), the norm determinationunit 203 computes a norm of the stored edge vector (S404). Then, thenorm determination unit 203 compares the computed norm with a norm ofthe maximum edge vector (S405). when the computed norm is larger thanhalf the norm of the maximum vector (Yes in S405), the normdetermination unit 203 computes an angle of the vector in the currentpixel position, and stores the angle in the memory (S406).

The aforementioned processes (S403 to S406) are performed on all of thecurrent pixels. When there is no next pixel, i.e., the processes areperformed on all of the current pixels (No in S407), the normdetermination unit 203 computes the angle of the edge by averaging theangles stored in the memory (S408).

The above processes for detecting an edge and determining its angle asshown in FIG. 10 are merely exemplary. The present invention is notlimited to any particular method for detecting edges or determiningtheir directions. In particular, the angle may also be determined from adirection of the maximum edge vector only or from a greater or smallernumber of neighboring vectors or other vectors along the edge.Furthermore, although a vector stored in the memory is defined as anedge vector in the description, it may be a gradient vector.

When the prediction mode selection unit 205 selects the edge predictionmode, the intra-prediction unit 112 generates a predicted block usingthe angle of the edge determined as indicated above, i.e., a directionof the edge as a prediction direction. Here, the intra-prediction unit112 may compute the angle of the edge. In such a case, theintra-prediction unit 112 includes processing units corresponding to theedge detection unit 201, the direction determination unit 202, and thenorm determination unit 203.

The same holds true for the image decoding apparatus 300 when computingthe angle of the edge. More specifically, the processing performed bythe edge detection unit 201, the direction determination unit 202, andthe norm determination unit 203 are respectively performed by the edgedetection unit 401, the direction determination unit 402, and the normdetermination unit 403.

Next, a process for generating a predicted block according to the edgeprediction mode of the embodiment, with reference to FIG. 11A to FIG.13.

FIGS. 11A, 11B, and 12 are schematic drawings illustrating the edgeprediction mode of the embodiment. According to this mode, the currentblock is predicted by extrapolating or interpolating previously codedand decoded video data along a certain direction. A direction of intraprediction is fixed in the conventional intra-prediction modes (FIG.1B), whereas the current block can be predicted along any direction inthe edge prediction mode. According to the embodiment, a direction forprediction is determined by detecting an edge in image data that hasbeen previously coded and decoded.

FIG. 11A illustrates a case where an edge direction 50 that has beendetected crosses an upper border of the current block 10. As illustratedin FIG. 11A, the current block 10 is predicted by extrapolating thepreviously coded and decoded video data, more specifically, by aligningthe reference pixels 20 along the detected edge direction 50 of thedetected edge. In other words, the intra-prediction unit 112 generates apredicted block using pixel values of the reference pixels 20 that havebeen determined according to the direction of the edge (or interpolatedvalues computed from one or more of the reference pixels 20) as pixelvalues (predicted values) of pixels included in the predicted block.

The intra-prediction unit 112 employs an linear interpolation among thereference pixels 20 for computing predicted values of the pixels inorder to preserve the sharpness of the edge. As illustrated in FIG. 11A,a position determined by a parallel translation of the reference pixelsaccording to the determined angle α does not match an integer pixelposition.

For each pixel to be predicted at a position (x, y), a referenceposition (x−δ_(x)) is determined depending on the angle α of the edgewhich is indicated by the edge direction 50. A reference value at thereference position is then computed by linearly interpolating 2surrounding integer reference pixels “a” and “b”. The computed referencevalue is then used as a predicted value “p” of a pixel to be predicted.The available reference pixels are the reference pixels 20 directlyadjacent to the current block 10.

The predicted value “p” is computed as a weighted sum of pixel values ofthe 2 reference pixels “a” and “b”, using Equation 3 below.p=w _(a) a+w _(b) b  [Equation 3]

Here, w_(a)=δ_(x)−floor (δ_(x)), w_(b)=ceil (δ_(x)), and δ_(x)=y cot α.

The horizontal increment δ_(x) is computed from the angle α of thedetected edge and the position (x, y) of the pixel that is to bepredicted within the block. A position (0, 0) is at the upper-leftcorner of the reference pixels 20 adjacent to the current block 10 inFIG. 11A. Thus, the predicted value “p” is indicated at a point (3, 2)in FIG. 11A.

An equation similar to Equation 3 is employed when a line through pixelsand parallel to the detected edge (an edge direction 51) crosses a leftborder, not the upper border of the current block 10. FIG. 11Billustrates a case where the edge direction 51 of the detected edgecrosses the left border of the current block 10. Here, a verticalincrement is indicated by δ_(y), and the predicted value “p” for theposition (x, y) of the pixel is computed using reference pixel values“c” and “d” according to Equation 4.p=w _(c) c+w _(d) d  [Equation 4]

Here, w_(c)=δ_(y)−floor (δ_(y)), w_(d)=ceil (δ_(y))−δ_(y), and δ_(y)=xtan α.

A position (0, 0) is at the upper-left corner of the reference pixels 20adjacent to the current block 10 in FIG. 11B. Thus, the predicted value“p” is indicated at a point (4, 4) in FIG. 11B.

As indicated in FIG. 12, there may be 2 relevant reference positions(“a” and “b”, and “c” and “d”), if a line through pixels and parallel tothe detected edge (an edge direction 52) crosses the left border of thecurrent block as well as its upper border or the upper border of a blockpositioned to the right of the current block. In this case, the currentblock 10 may be predicted by interpolating previously coded and decodedvideo data rather than by an extrapolation, so that both referencepositions are used for prediction. The predicted value is the result ofa linear interpolation of the interpolated pixel values at the 2reference positions.

Specifically, the predicted value “p” may be computed as a weighted sumof the 4 reference pixel values “a”, “b”, “c”, and “d”, for instance, asindicated in Equation 5.

$\begin{matrix}{p = {{\frac{w_{up}}{w_{up} + w_{left}}\left( {{w_{a}a} + {w_{b}b}} \right)} + {\frac{w_{up}}{w_{up} + w_{left}}\left( {{w_{c}c} + {w_{d}d}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Here, w_(up)=N−y, w_(left)=N−x, and N=4 or N=8, where N depends on ablock size.

Another possibility would be to compute the real interpolation betweenthe 2 reference positions, i.e., w_(up) and w_(left) being a Euclideandistance between the 2 reference positions.

As described above, the intra-prediction unit 112 predicts the currentblock by extrapolating or interpolating previously generated (coded anddecoded) decoded image data (i.e., reference pixels (also referred to asdecoded pixels) along a direction of an detected edge. The decoded imagedata corresponds to pixels included in a block adjacent to the currentblock. The extrapolation or interpolation means a linear extrapolationor interpolation.

More specifically, the intra-prediction unit 112 computes a weighted sumof at least 2 reference pixel values for each predicted pixel includedin a predicted block (refer to Equations 3, 4, and 5), and generates thepredicted block by extrapolating or interpolating the computed weightedsum. The weights (w_(a), w_(b), w_(c), and w_(d)) are determinedaccording to the direction of the edge. For example, a larger value isassigned to each of the weights, as a distance from one of the referencepixels respectively corresponding to the weights to a line that crossesthe predicted pixels and is directed to an edge (such as the edgedirection 50) is smaller.

FIG. 13 is a flowchart illustrating a method for computing a predictedvalue (i.e., generating a predicted block) according to the edgeprediction mode in the embodiment.

First, the intra-prediction unit 112 determines an angle of an edge inaccordance with the processes described with reference to FIG. 10(S501), for instance. The following steps are performed on all pixels ofthe current block to be predicted (S502).

The intra-prediction unit 112 determines whether or not there is areference pixel positioned upward of the current pixel along a directiondefined by the computed angle, for example, as illustrated in FIG. 11A(S503). When there is such a reference pixel upward (Yes in S503), theintra-prediction unit 112 computes an upper reference value r_(up) usingEquation 3 (S504). Here, the intra-prediction unit 112 computes theupper reference value r_(up) by replacing “p” with r_(up) in Equation 3.

Next, the intra-prediction unit 112 determines whether or not there is areference pixel to the left of the current pixel, along the directiondefined by the computed angle, for example, as illustrated in FIG. 11B(S505). When there is such a reference pixel to the left (Yes in S505),the intra-prediction unit 112 computes a left reference value r_(left)using Equation 4 (S506). Here, the intra-prediction unit 112 computesthe left reference value r_(left) by replacing “p” with r_(left).

Then, the intra-prediction unit 112 computes the predicted value “p” ofthe current pixel according to the availability of upper and leftreference values (S507). When both reference values are available, thepredicted value “p” is computed byp=(w_(up)r_(up)+w_(left)r_(left))/(w_(up)+w_(left)). When only one ofthe upper reference value and the left reference value is available, thepredicted value “p” is set to p=r_(up) and p=r_(left), respectively.When neither of them is available, the predicted value “p” is computedby averaging pixel values of adjacent pixels that have been previouslypredicted, in other words, using Equation 6.p(x,y)=1/3[p(x−1,y−1)+p(x−1,y)+p(x,y−1)]  [Equation 6]

Here, p(x, y) denotes a predicted value at a position (x, y) in Equation6.

When there is a next pixel (Yes in S508), the processes (S503 to S507)are repeated. More specifically, the processes are executed untilpredicted values for all pixels included in the current block arecomputed.

As described above, the image coding apparatus 100 of the embodiment cangenerate a predicted block by computing predicted values along adirection of an detected edge, when the edge prediction mode isselected.

The same holds true for the image decoding apparatus 300 when computingpredicted values. More specifically, the processing performed by theintra-prediction unit 112 is executed by the intra-prediction unit 305.

Furthermore, although a predicted block is generated along a direction,the predicted block may be generated along 2 or more directions,according to the embodiment.

FIG. 14 schematically illustrates that predicted pixels are predictedalong 2 or more directions according to the embodiment. For example,when two strong edges 61 and 62 having directions different from eachother but both pointing to the current block are detected in twonon-adjacent pixels 63 and 64, pixel values may be predicted byextrapolating or interpolating the previously decoded reference pixels20 along one of two directions of edges, or more preferably, along adirection obtained by combining edge directions 65 and 66. For example,the edge directions 65 and 66 may be combined by computing a weightedsum as in Equation 5. The horizontal increment δ_(x) and the verticalincrement δ_(y), however, will be computed from the edge directions 65and 66 (angles) of the first edge and the second edge, respectively.

The present invention is limited neither with respect to the number ofedges and directions that are employed for prediction nor with respectto the way in which the individual results are combined.

In order to preserve the sharpness of each of these edges, the weightsfor use in computing a weighted sum of reference pixel values may alsobe computed according to a distance from each predicted pixel to bepredicted to an extension of each of the detected edges. For example, alarger value is assigned to each of the weights, as a distance from oneof the reference pixels (“a”, “b”, “c”, and “d” in FIG. 14) respectivelycorresponding to the weights to an extension of one of at least 2detected edges is smaller.

In particular, the weights w_(left) and w_(up) may be set to 1 and 0,respectively, for all pixels that are crossed by an edge (or anextension of an edge) detected in a block to the left of the currentblock. Furthermore, the weights w_(left) and w_(up) may be set to 1 and0, respectively, for all pixels that are crossed by an edge (or anextension of an edge) detected in a block above the current block.Specifically, a larger value is assigned to each of the weights, as adistance from the reference pixels (“a”, “b”, “c”, and “d” in FIG. 14)respectively corresponding to the weights to a decoded block from whichat least one of the two edges is detected is smaller. The weights forthe remaining pixels, i.e., the pixels that are not crossed by any ofdetected edges (or extensions of edges) may be set, depending on adistance from a predicted pixel, as described above.

Next, a decoding process using intra prediction according to theembodiment will be described with reference to FIG. 15. FIG. 15 is aflowchart illustrating a process for decoding image data and video datausing intra prediction according to the embodiment.

The decoding process in FIG. 15 is executed for each macroblock of 16×16pixels (S601). First, a partition size is read from an input bitstream,i.e., is decoded by a statistical decoder (S602). Then, the decodingprocesses are executed for each block of the current macroblockaccording to the partition size (S603).

The prediction mode of the current block and its residual informationindicating the coded prediction error are read from the input bitstream(S604). The read prediction mode is outputted to the intra-predictionunit 305 or the motion compensation unit 306, through the control unit307 and the switch 308. The residual information is outputted to theinverse quantization unit 301.

Next, the intra-prediction unit 305 or the motion compensation unit 306generates a predicted block corresponding to the current block,according to the read prediction mode (S605). The process, inparticular, executed by the intra-prediction unit 305 in the process forgenerating the predicted block will be described in further detail withreference to FIG. 16.

The inverse quantization unit 301 inverse-quantizes the residualinformation, and outputs the inverse-quantized residual information tothe inverse frequency transform unit 302. Then, the inverse frequencytransform unit 302 performs inverse frequency transformation on theinverse-quantized residual information (S606). The decoded differentialimage generated through the inverse frequency transformation isoutputted to the adding unit 303.

The adding unit 303 reconstructs the current block by adding thepredicted block and the decoded differential image (S607). The decodedimage generated by the adding unit 303 is stored in the frame memory304, and is used by the intra-prediction unit 305 or the motioncompensation unit 306 as a reference picture.

Then, when there is a next block (Yes in S608), the processes fromreading the prediction mode and the prediction residual (S604) are thenrepeated on the next block. When there is no next block (No in S608) butthere is a next macroblock (Yes in S609), the processes from reading thepartition size (S602) are repeated on the next macroblock.

Next, an intra-prediction process for the image decoding apparatus 300according to the embodiment will be described with reference to FIG. 16.FIG. 16 is a flowchart illustrating the intra-prediction processaccording to the embodiment. Here, the intra-prediction unit 305performs the intra-prediction process in FIG. 16 when generating apredicted block (S605 in FIG. 15).

The prediction mode determination unit 404 determines whether or not theprediction mode is equal to 2, i.e., the prediction mode indicates theDC prediction mode or the edge prediction mode (S701). When theprediction mode is equal to 2 (Yes in S701), the edge detection unit401, the direction determination unit 402, and the norm determinationunit 403 perform edge detection (FIG. 7) (S702).

When the edge is detected (Yes in S703), the prediction modedetermination unit 404 selects the edge prediction mode, and thepredicted block generation unit 405 generates the predicted blockaccording to the edge prediction mode (S704). The processes forgenerating the predicted block are as described with reference to FIG.13.

When the edge is not detected (No in S703), the prediction modedetermination unit 404 selects the DC prediction mode, and the predictedblock generation unit 405 generates the predicted block according to theconventional DC prediction mode (S705).

When the prediction mode is different from 2 (No in S701), theprediction mode determination unit 404 selects one of the conventionaldirectional prediction modes indicated by the prediction mode, and thepredicted block generation unit 405 generates the predicted blockaccording to the conventional directional prediction mode (S706).

As described above, the image decoding apparatus 300 of the embodimentdetermines a prediction mode of the current block, and performs edgedetection when the intra-prediction mode is equal to 2, i.e., theprediction mode indicates one of the DC prediction mode and the edgeprediction mode. Furthermore, the image decoding apparatus 300determines that the prediction mode is the edge prediction mode when anedge is detected, and that the prediction mode is the DC prediction modewhen an edge is not detected.

Thereby, a bitstream coded according to the edge prediction mode can beaccurately decoded. Furthermore, since edge detection is not alwaysperformed but limited, the processing load of the image decodingapparatus 300 can be reduced, and spending much time on the decodingprocess can be prevented.

As described above, the image coding apparatus 100 and the imagedecoding apparatus 300 of the embodiment can employ the new edgeprediction mode when the intra prediction is performed. Thereby, when apicture including a sharp linear edge and to be coded is coded, highercoding efficiency can be achieved.

Furthermore, the image coding apparatus 100 of the embodiment codesinformation indicating the new edge prediction mode as informationindicating the DC prediction mode. In other words, the image codingapparatus 100 employs the very same code for signaling the conventionalDC intra-prediction mode and the new intra-prediction mode. Thereby, theamount of coded data is not increased along with the use of the newintra-prediction mode.

Furthermore, the image coding apparatus 100 of the embodiment does notcode information necessary for the edge prediction mode, such as adirection of an edge so as to detect a direction of an edge from blockssurrounding the current block. Furthermore, the image decoding apparatus300 includes a processing unit that performs edge detection as in theimage coding apparatus 100, so that there is no need for signalinginformation necessary for the edge prediction mode.

Thus, a codec apparatus including the image coding apparatus 100 and theimage decoding apparatus 300 of the embodiment does not need anyadditional signaling overhead for using the new intra-prediction mode.

FIGS. 17A, 17B, and 18 show diagrams for describing advantages of theimage coding apparatus 100 of the embodiment. FIG. 17A shows arelationship between a Peak Signal-to-Noise Ratio (PSNR) and a bitratewhen the first 10 frames of a sequence Foreman (Common IntermediateFormat (resolution of 352×288) abbreviated as CIF) are coded. FIG. 17Bshows a relationship between a PSNR and a bitrate when a sequenceSpinCalendar is coded. Furthermore, FIG. 18 is a chart indicating thepercentage of blocks obtained by coding different sequences using theedge prediction mode, and the bitrate reduction.

For example, when the sequence SpinCalendar was coded, the bitratereduction was over 8%, showing a significant improvement in codingefficiency of sequences including lots of sharp edges. It turned outthat the results for coding the sequence Foreman were better when codingthe first 10 frames of the sequence rather than coding 300 frames (allsequences) (6.44% of bitrate reduction). The reason is that the sequenceForeman includes lots of sharp linear edges at the beginning of thesequence. In these frames, the edge prediction mode is often selected(for about 15% of the blocks on average) and the edges are betterpredicted. On the other hand, there are very few edges at the end of thesequence. In these frames, the edge prediction mode is almost never used(for only about 3% to 4% of the blocks on average). That is why the meanbitrate reduction for the whole sequence is 3.37%.

These results do also show that the performance of the edge predictionis highly dependent on content of an image. For images with very fewedges or blurred edges, the edge prediction mode is almost never used.For such sequences, there is no advantage as compared to theconventional coding method, but there is no loss either, because the DCprediction mode is almost always used in the prediction mode set. Suchimages are coded with the 9 conventional prediction modes of H.264.

The following table contains the results on the high precision edgeprediction technique in terms of bitrate reduction and PSNR improvementfor the sequences simulated.

TABLE 1 Average bitrate Average PSNR reduction improvement Foreman (CIF,3.37% 0.20 dB resolution of 352 × 288) Paris (CIF) 1.67% 0.15 dBToysAndCalendar 2.62% 0.16 dB (VGA, resolution of 640 × 480)TrainStation (VGA) 4.62% 0.23 dB BigShips (720 p, 1.97% 0.11 dBresolution of 1280 × 720) SpinCalendar 8.51% 0.51 dB (720 p)

As shown in Table 1, the results can differ a lot between differentsequences. There is no linear relationship between the frequency ofusing the edge prediction mode and the final gain, but the best gainsare obtained for the images containing lots of edges easily predictedwith the edge prediction mode.

Although an image coding apparatus and an image decoding apparatusaccording to an aspect of the present invention are describedhereinbefore, the present invention is not limited to these in theembodiment. The modifications conceived by those skilled in the art areincluded within the scope of the present invention, as long as they arenot departing from the purport of the present invention.

According to the above embodiment of the present invention, gradientsare computed using the Sobel operators to detect edges. The presentinvention, however, is not limited in this respect. Instead, any edgedetection tool may be employed provided that the directionality of thedetected edge is computed. The Sobel operators are only one example of apossible edge detection technique. For example, the Prewitt operators asindicated in Equation 7 may be used.

$\begin{matrix}{{{Prewitt}_{x} = \begin{bmatrix}{- 1} & 0 & 1 \\{- 1} & 0 & 1 \\{- 1} & 0 & 1\end{bmatrix}},{{Prewitt}_{y} = \begin{bmatrix}{- 1} & {- 1} & {- 1} \\0 & 0 & 0 \\1 & 1 & 1\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

Furthermore, gradients on blocks that have been low-pass filtered (usinga low pass filter abbreviated as LPF) may be computed. Morespecifically, gradients may be computed using an operator obtained byconvolving the Sobel operators or the Prewitt operators, and a filtercoefficient included in an LPF. Thus, the edge detection unit 201 or 401may compute gradients, for example, by selecting one filter out of (i)only the Sobel operators, (ii) only the Prewitt operators, (iii) asynthetic filter synthesized from the Sobel operators and an LPF, and(IV) a synthetic filter synthesized from the Prewitt operators and anLPF. A filter is selected, for example, based on an instruction from theuser outside or a characteristic of an current block. When the currentblock includes a large amount of noise, i.e., the current block is notflat, for example, a filter including an LPF is used.

Furthermore, although the norm determination unit 203 of the imagecoding apparatus 100 or the norm determination unit 403 of the imagedecoding apparatus 300 according to the embodiment determines a normusing a predetermined threshold, the norm may be changed per sequence,picture, slice, or block. In this case, the predetermined thresholdneeds to be included in a bitstream and transmitted to a decoder. Forexample, information indicating a threshold may be included in asequence parameter set (SPS), a picture parameter set (PPS), or a sliceheader (SH) on a per unit basis for determining a threshold.

FIG. 19 is a block diagram illustrating an example of a configuration ofan image coding apparatus 500 that differs from the image codingapparatus 100 of the embodiment. The image coding apparatus 500 in FIG.19 differs from the image coding apparatus 100 in FIG. 2 in including: avariable length coding unit 505 in replacement of the variable lengthcoding unit 105; a reference picture memory 509 in replacement of thereference picture memory 109; an intra-prediction mode determinationunit 510 in replacement of the intra-prediction mode determination unit110, and additionally a threshold determination unit 519. The samefeatures as those of the image coding apparatus 100 in FIG. 2 will beomitted, and thus the differences will be mainly described hereinafter.

Furthermore, the image coding apparatus 500 may receive an edgedetection flag from outside as illustrated in FIG. 19. The edgedetection flag is a flag for switching between validity and invalidityof the edge detection. For example, validity and invalidity of the edgedetection can be switched per sequence, picture, or slice.

The variable length coding unit 505 adds a threshold determined by thethreshold determination unit 519 and the edge detection flag to abitstream, in addition to the operations by the variable length codingunit 105. Information indicating a threshold and an edge detection flagare included, for example, in a SPS, PPS, and a SH.

The reference picture memory 509 stores the decoded images LDs generatedby the adding unit 108. Then, the reference picture memory 509 outputs,as reference pictures, the decoded images LDs not only stored in theintra-prediction mode determination unit 510, the motion estimation unit111, the intra-prediction unit 112, and the motion compensation unit 113but also stored in the threshold determination unit 519.

The intra-prediction mode determination unit 510 receives an edgedetection flag, in addition to the operations by the intra-predictionmode determination unit 110. Then, the intra-prediction modedetermination unit 510 switches between validity and invalidity of theedge detection, according to the received edge detection flag. Morespecifically, when an edge detection flag indicates validity of the edgedetection, the intra-prediction mode determination unit 510 performs thesame processing as the intra-prediction mode determination unit 110.More specifically, when an edge detection flag indicates invalidity ofthe edge detection, the intra-prediction mode determination unit 510always sets the intra-prediction mode set including the conventional DCprediction mode and the 8 prediction directions. In other words, theintra-prediction mode determination unit 510 does not perform edgedetection.

Furthermore, the intra-prediction mode determination unit 510 determinesa norm using the threshold determined by the threshold determinationunit 519. The specific description of the norm determination process isomitted because it is previously described.

The threshold determination unit 519 reads a reference picture from thereference picture memory 509, and determines a threshold by computing acharacteristic value of the read reference picture. The characteristicvalue of a reference picture is, for example, a variance and intensityof the reference picture in which edge detection is performed. Forexample, a larger value is assigned to a threshold for use indetermining a norm, as a variance becomes larger. Since a largervariance indicates that a reference picture includes a large amount ofnoise, it is highly likely that an edge is wrongly detected. Thus, useof a larger threshold in determining a norm can reduce a possibilitythat an edge is wrongly detected.

Furthermore, the threshold determination unit 519 may determine athreshold based on a result of the edge detection on the previouspicture. For example, a larger value is assigned to a threshold, as moreedges are detected along a direction different from the direction of theprevious picture. The edges detected along the different directions arelarger in number, because there is a possibility that an edge is notaccurately detected due to influence of noise. Thus, assigning a largervalue to a threshold for use in determining a norm can reduce thepossibility that an edge is wrongly detected.

As described above, the image coding apparatus 500 in FIG. 19 canadaptively change a threshold. Thereby, the image coding apparatus 500can determine a more suitable threshold, and generate a more suitablepredicted block. Thus, coded distortion and others can be reduced.

Furthermore, since the present invention can invalidate edge detectionon a flat image that cannot sufficiently enjoy the advantage of thepresent invention, the processing load of the image coding apparatus 100can be reduced, and coding process can be sped up.

Furthermore, the edge detection flag is transmitted to a decoder,preventing the decoder from performing edge detection on a bitstream inwhich the edge detection has not yet been performed.

The image decoding apparatus 300 of the embodiment needs to store, inthe frame memory 304, not only the reference pixels 20 in FIG. 1 butalso data of all pixels included in the blocks including the referencepixels 20 in order to perform the edge detection. Thus, the imagedecoding apparatus 300 may compute gradients at the same time whendecoding surrounding blocks, i.e., obtaining pixel values of referencepixels necessary for computing the gradients, and store only the resultsof the computation in the frame memory 304. Here, the image decodingapparatus 300 can further save memory resources by storing only a norm(or gradient value) beyond a threshold, for example. Alternately, theimage decoding apparatus 300 may store only the largest norm in eachblock and its pixel position.

In this case, as illustrated in FIG. 20, pixels included in a pixelgroup 72 immediately above the current block 10 are stored as referencepixels to be referred to when a predicted block is generated, in thesame manner as described for the conventional technique and theembodiment. Furthermore, not pixel values included in the surroundingblocks 30 but a norm (or gradient value) that is computed for pixelsincluded in a pixel group 71 subject to computation of a direction of anedge is stored in the frame memory 304 or a memory and other mediaincluded in the norm determination unit 403.

Thereby, the memory resources of the image decoding apparatus 300 can beeffectively used.

Furthermore, the method for computing predicted values may be replacedwith any method duly in consideration of the detected direction of anedge, within the scope of the present invention. More specifically,reference pixels larger than the described reference pixels in number orother reference pixels may be used in replacement for reference pixelsadjacent to the current block to be predicted.

Further, the present invention is not limited to the H.264 video codingstandard or to the conventional intra-prediction mode set describedabove. In fact, the inventive edge prediction mode can also be used inany block-based video encoder using spatial prediction. In particular,the edge prediction mode may be used as an additional mode in H.264/AVCrather than as a replacement of the DC prediction mode.

Further, the inventive edge prediction mode may also be employed inconjunction with a signaling mechanism that is different from the abovedescribed combined signaling of the edge prediction mode and the DCprediction mode. In particular, the edge prediction mode may be signaledby a dedicated code word independent of the DC prediction mode or incombination with one or more of the predefined directional predictionmodes.

Further, the present invention is not limited to video codingapplications but may also be used for block-based still image coding.

Furthermore, the present invention can be implemented not only as animage coding apparatus, an image decoding apparatus, and a methodthereof but also as a program causing a computer to execute an imagecoding method and an image decoding method of the embodiment.Furthermore, the present invention can be implemented as a recordingmedium that records the program, such as a computer-readable CD-ROM.Furthermore, the present invention can be implemented as information,data, or a signal each indicating the program. Furthermore, theseprogram, information, data, and signal may be distributed through acommunication network, such as the Internet.

A part or all of the constituent elements included in the respectiveimage coding apparatus and image decoding apparatus may be configuredfrom a single System-Large-Scale Integration (LSI). The System LSI is asuper-multi-function LSI manufactured by integrating constituent unitson one chip, and is specifically a computer system configured byincluding a microprocessor, a ROM, a RAM, and so on.

Summarizing, the present invention relates to coding and decoding ofimage data and video data, and in particular to a new spatial predictionmode for predicting the current block from image data of previouslycoded and decoded blocks. According to this prediction mode, blockscontaining sharp edges may be predicted more faithfully by taking aprecise direction of an edge into account. Moreover, the new predictionmode of the present invention does not require any additional signalingoverhead, because (i) the direction of the edge can be predicted frompreviously coded blocks and (ii) the new prediction mode can be replacedwith the conventional DC prediction mode for blocks containing sharpedges.

INDUSTRIAL APPLICABILITY

The image coding method and image decoding method of the presentinvention have an advantage of enhancing coding efficiency, and isapplicable to, for example, a digital camera, a digital television, anda Blu-ray Disc (BD) recorder.

The invention claimed is:
 1. A method for coding image data on ablock-by-block basis, said method comprising: partitioning the imagedata into a plurality of blocks; generating a predicted block bypredicting a current block that is one of the plurality of blocks;computing a difference between the current block and the predictedblock; coding the difference computed in said computing; decoding thedifference coded in said coding; and adding the difference decoded insaid decoding to the predicted block to generate a decoded block,wherein said generating includes: detecting an edge in a previouslydecoded block corresponding to a block adjacent to the current block;and extrapolating or interpolating previously decoded image data along adirection of the edge detected in said detecting to generate thepredicted block, the previously decoded image data corresponding to apixel included in the block adjacent to the current block, and wherein,in said coding, a mode indicator is coded, the mode indicator indicatingthat the edge in the previously decoded block is to be detected togenerate the predicted block.
 2. The method for coding image dataaccording to claim 1, wherein the previously decoded image data islinearly extrapolated or interpolated in said extrapolating orinterpolating to generate the predicted block.
 3. The method for codingimage data according to claim 2, wherein a weighted sum of at least 2pixel values of a plurality of decoded pixels included in the previouslydecoded image data is computed for each predicted pixel included in thepredicted block, and the computed weighted sum is extrapolated orinterpolated to generate the predicted block in said extrapolating orinterpolating, and wherein weights for computing the weighted sum aredetermined according to the direction of the edge detected in saiddetecting.
 4. The method for coding image data according to claim 3,wherein a larger value is assigned to each of the weights, as a distancefrom one of the plurality of decoded pixels to one of the predictedpixels is smaller, such that each of the plurality of decoded pixelsrespectively corresponds to one of the weights for computing theweighted sum.
 5. The method for coding image data according to claim 2,wherein at least 2 edges having directions different from each other aredetected in said detecting, and wherein pixel values of a plurality ofdecoded pixels for extrapolation or interpolation along the directionsof the at least 2 edges are combined for each of predicted pixelsincluded in the predicted block, and the combined pixel values areextrapolated or interpolated to generate the predicted block in saidextrapolating or interpolating.
 6. The method for coding image dataaccording to claim 5, wherein a weighted sum is computed for each of thepredicted pixels by multiplying each of the weights by a correspondingone of the pixel values of the plurality of decoded pixels, and thecomputed weighted sum is extrapolated or interpolated to generate thepredicted block in said extrapolating or interpolating, and wherein alarger value is assigned to each of the weights, as a distance from oneof the plurality of decoded pixels to the previously decoded block inwhich one of the at least 2 edges is detected is smaller, such that eachof the plurality of decoded pixels respectively corresponds to one ofthe weights.
 7. The method for coding image data according to claim 6,wherein a larger value is assigned to each of the weights, as a distancefrom one of the plurality of decoded pixels to an extension of one ofthe at least 2 edges is smaller, such that each of the plurality ofdecoded pixels respectively corresponds to one of the weights.
 8. Themethod for coding image data according to claim 1, wherein saidgenerating further includes selecting one of a plurality of predictionmodes including an edge prediction mode, wherein, when the edgeprediction mode is selected, the previously decoded image data isextrapolated or interpolated along the direction of the edge detected insaid detecting to generate the predicted block in said extrapolating orinterpolating, and wherein, when the edge prediction mode is selected,in said coding, the mode indicator is coded, the mode indicatorindicating that the edge in the previously decoded block is to bedetected to generate the predicted block.
 9. The method for coding imagedata according to claim 8, wherein the plurality of prediction modesfurther includes a DC prediction mode and a plurality of predefineddirectional prediction modes, and wherein, in said extrapolating orinterpolating, pixel values of the previously decoded image data areaveraged to generate the predicted block when the DC prediction mode isselected, and the previously decoded image data is extrapolated along adirection corresponding to a selected one of the plurality ofdirectional prediction modes to generate the predicted block, when theone of the plurality of directional prediction modes is selected. 10.The method for coding image data according to claim 9, wherein, in saidselecting: (i) one of the edge prediction mode and the plurality ofdirectional prediction modes is selected when the edge is detected insaid detecting; and (ii) one of the DC prediction mode and the pluralityof directional prediction modes is selected when the edge is notdetected in said detecting.
 11. The method for coding image dataaccording to claim 10, wherein, in said coding: (i) when one of theplurality of directional prediction modes is selected in said selecting,a mode indicator indicating the selected one of plurality of directionalprediction modes is coded; and (ii) when one of the DC prediction modeand the edge prediction mode is selected, a mode indicator indicatingthat the selected one of the prediction modes is one of the DCprediction mode and the edge prediction mode is coded.
 12. The methodfor coding image data according to claim 9, wherein, in said selecting:one of the edge prediction mode and the plurality of directionalprediction modes is selected when the edge is detected in said detectingand the direction of the edge detected in said detecting points to thecurrent block; and one of the DC prediction mode and the plurality ofdirectional prediction modes is selected when (i) the edge is notdetected in said detecting, or (ii) the edge is detected in saiddetecting and the direction of the edge detected in said detecting doesnot point to the current block.
 13. The method for coding image dataaccording to claim 1, wherein said detecting includes: computing agradient vector for each of a plurality of pixels included in thepreviously decoded block corresponding to the block adjacent to thecurrent block; determining whether at least one of the gradient vectorscomputed in said computing satisfies conditions that (i) a norm of theat least one of the gradient vectors exceeds a predetermined thresholdand (ii) a vector having a direction perpendicular to a direction of theat least one of the gradient vectors points to the current block; anddetecting an edge having the direction perpendicular to the direction ofat least one of the gradient vectors when the at least one of thegradient vectors computed in said computing satisfies the conditions insaid determining.
 14. The method for coding image data according toclaim 13, wherein, in said detecting of the edge having the direction,the direction of the edge is determined based on a direction of agradient vector having a largest norm, the gradient vector having thelargest norm corresponding to one of the at least one of the gradientvectors satisfying the conditions.
 15. The method for coding image dataaccording to claim 13, wherein, in said detecting of the edge having thedirection, the direction of the edge is determined based on a directionobtained by averaging directions of the at least one of the gradientvectors satisfying the conditions.
 16. A method for decoding image dataincluding a coded prediction residual on a block-by-block basis, saidmethod comprising: decoding the prediction residual of a current block;generating a predicted block by predicting the current block; and addingthe predicted block generated in said generating to the predictionresidual decoded in said decoding to generate a decoded block, whereinthe image data further includes a mode indicator, and wherein, when themode indicator indicates that an edge in a previously decoded block isto be detected to generate the predicted block, said generatingincludes: detecting the edge in the previously decoded blockcorresponding to a block adjacent to the current block; andextrapolating or interpolating previously decoded image data along adirection of the edge detected in said detecting to generate thepredicted block, the previously decoded image data corresponding to apixel included in the block adjacent to the current block.
 17. Themethod for decoding image data according to claim 16, wherein thepreviously decoded image data is linearly extrapolated or interpolatedin said extrapolating or interpolating to generate the predicted block.18. The method for decoding image data according to claim 17, wherein aweighted sum of at least 2 pixel values of a plurality of decoded pixelsincluded in the previously decoded image data is computed for eachpredicted pixel included in the predicted block, and the computedweighted sum is extrapolated or interpolated to generate the predictedblock in said extrapolating or interpolating, and wherein weights forcomputing the weighted sum are determined according to the direction ofthe edge detected in said detecting.
 19. The method for decoding imagedata according to claim 18, wherein a larger value is assigned to eachof the weights, as a distance from one of the plurality of decodedpixels to one of the predicted pixels is smaller, such that each of theplurality of decoded pixels respectively corresponds to one of theweights.
 20. The method for decoding image data according to claim 17,wherein at least 2 edges having directions different from each other aredetected in said detecting, and wherein pixel values of a plurality ofdecoded pixels for extrapolation or interpolation along the directionsof the at least 2 edges are combined for each of predicted pixelsincluded in the predicted block, and the combined pixel values areextrapolated or interpolated to generate the predicted block in saidextrapolating or interpolating.
 21. The method for decoding image dataaccording to claim 20, wherein a weighted sum is computed for each ofthe predicted pixels by multiplying each of the weights by acorresponding one of the pixel values of the plurality of decodedpixels, and the computed weighted sum is extrapolated or interpolated togenerate the predicted block in said extrapolating or interpolating, andwherein a larger value is assigned to each of the weights, as a distancefrom one of the plurality of decoded pixels to the previously decodedblock in which one of the at least 2 edges is detected is smaller, suchthat each of the plurality of decoded pixels respectively corresponds toone of the weights.
 22. The method for decoding image data according toclaim 21, wherein a larger value is assigned to each of the weights, asa distance from one of the plurality of decoded pixels to an extensionof one of the at least 2 edges is smaller, such that each of theplurality of decoded pixels respectively corresponds to one of theweights.
 23. The method for decoding image data according to claim 16,wherein said generating further includes selecting one of a plurality ofprediction modes including an edge prediction mode, and wherein, whenthe edge prediction mode is selected, the previously decoded image datais extrapolated or interpolated along the direction of the edge detectedin said detecting to generate the predicted block in said extrapolatingor interpolating.
 24. The method for decoding image data according toclaim 23, wherein the plurality of prediction modes further includes aDC prediction mode and a plurality of predefined directional predictionmodes, and wherein, in said extrapolating or interpolating, pixel valuesof the previously decoded image data are averaged to generate thepredicted block when the DC prediction mode is selected, and thepreviously decoded image data is extrapolated along a directioncorresponding to a selected one of the plurality of directionalprediction modes to generate the predicted block, when the one of theplurality of directional prediction modes is selected.
 25. The methodfor decoding image data according to claim 24, wherein, in saidselecting: (i) one of the edge prediction mode and the plurality ofdirectional prediction modes is selected when the edge is detected insaid detecting; and (ii) one of the DC prediction mode and the pluralityof directional prediction modes is selected when the edge is notdetected in said detecting.
 26. The method for decoding image dataaccording to claim 25, wherein in said selecting, (i) the edgeprediction mode is selected when the mode indicator indicates that aprediction mode to be selected is one of the DC prediction mode and theedge prediction mode and the edge is detected in said detecting, and(ii) the DC prediction mode is selected when the mode indicatorindicates that the prediction mode to be selected is one of the DCprediction mode and the edge prediction mode and the edge is notdetected in said detecting.
 27. The method for decoding image dataaccording to claim 24, wherein, in said selecting: one of the edgeprediction mode and the plurality of directional prediction modes isselected when the edge is detected in said detecting and the directionof the edge detected in said detecting points to the current block; andone of the DC prediction mode and the plurality of directionalprediction modes is selected when (i) the edge is not detected in saiddetecting, or (ii) the edge is detected in said detecting and thedirection of the edge detected in said detecting does not point to thecurrent block.
 28. The method for decoding image data according to claim16, wherein said detecting includes: computing a gradient vector foreach of a plurality of pixels included in the previously decoded blockcorresponding to the block adjacent to the current block; determiningwhether at least one of the gradient vectors computed in said computingsatisfies conditions that (i) a norm of the at least one of the gradientvectors exceeds a predetermined threshold and (ii) a vector having adirection perpendicular to a direction of the at least one of thegradient vectors points to the current block; and detecting an edgehaving the direction perpendicular to the direction of at least one ofthe gradient vectors when the at least one of the gradient vectorscomputed in said computing satisfies the conditions in said determining.29. The method for decoding image data according to claim 28, wherein,in said detecting of the edge having the direction, the direction of theedge is determined based on a direction of a gradient vector having alargest norm, the gradient vector having the largest norm correspondingto one of the at least one of the gradient vectors satisfying theconditions.
 30. The method for decoding image data according to claim28, wherein, in said detecting of the edge having the direction, thedirection of the edge is determined based on a direction obtained byaveraging directions of the at least one of the gradient vectorssatisfying the conditions.
 31. An image coding apparatus that codesimage data on a block-by-block basis, said image coding apparatuscomprising: a partitioning unit configured to partition the image datainto a plurality of blocks; a generating unit configured to generate apredicted block by predicting a current block that is one of theplurality of blocks; a computing unit configured to compute a differencebetween the current block and the predicted block; a coding unitconfigured to code the difference computed by said computing unit; adecoding unit configured to decode the difference coded by said codingunit; and an adding unit configured to add the difference decoded bysaid decoding unit to the predicted block to generate a decoded block,wherein said generating unit includes: a detecting unit configured todetect an edge in a previously decoded block corresponding to a blockadjacent to the current block; and an extrapolating or interpolatingunit configured to extrapolate or interpolate previously decoded imagedata along a direction of the edge detected by said detecting unit togenerate the predicted block, the previously decoded image datacorresponding to a pixel included in the block adjacent to the currentblock, and wherein said coding unit is further configured to code a modeindicator, the mode indicator indicating that the edge in the previouslydecoded block is to be detected to generate the predicted block.
 32. Animage decoding apparatus that decodes image data including a codedprediction residual on a block-by-block basis, said image decodingapparatus comprising: a decoding unit configured to decode theprediction residual of a current block; a generating unit configured togenerate a predicted block by predicting the current block; and anadding unit configured to add the predicted block generated by saidgenerating unit to the prediction residual decoded by said decoding unitto generate a decoded block, wherein the image data further includes amode indicator, and wherein said generating unit includes: a detectingunit configured to detect an edge in a previously decoded blockcorresponding to a block adjacent to the current block, when the modeindicator indicates that the edge in the previously decoded block is tobe detected to generate the predicted block; and an extrapolating orinterpolating unit configured to extrapolate or interpolate previouslydecoded image data along a direction of the edge detected by saiddetecting unit to generate the predicted block, when the mode indicatorindicates that the edge in the previously decoded block is to bedetected to generate the predicted block, the previously decoded imagedata corresponding to a pixel included in the block adjacent to thecurrent block.
 33. A codec apparatus, comprising: the image codingapparatus according to claim 31; and an image decoding apparatuscomprising: a decoding unit configured to decode the prediction residualof a current block; a generating unit configured to generate a predictedblock by predicting the current block; and an adding unit configured toadd the predicted block generated by said generating unit to theprediction residual decoded by said decoding unit to generate a decodedblock, wherein the image data further includes a mode indicator, andwherein said generating unit of said image decoding apparatus includes:a detecting unit configured to detect an edge in a previously decodedblock corresponding to a block adjacent to the current block, when themode indicator indicates that the edge in the previously decoded blockis to be detected to generate the predicted block; and an extrapolatingor interpolating unit configured to extrapolate or interpolatepreviously decoded image data along a direction of the edge detected bysaid detecting unit of said image decoding apparatus to generate thepredicted block, when the mode indicator indicates that the edge in thepreviously decoded block is to be detected to generate the predictedblock, the previously decoded image data corresponding to a pixelincluded in the block adjacent to the current block.
 34. Anon-transitory computer-readable recording medium on which a program isrecorded, the program causing a computer to execute a method for codingimage data on a block-by-block basis, the method including: partitioningthe image data into a plurality of blocks; generating a predicted blockby predicting a current block that is one of the plurality of blocks;computing a difference between the current block and the predictedblock; coding the difference computed in the computing; decoding thedifference coded in the coding; and adding the difference decoded in thedecoding to the predicted block to generate a decoded block, wherein thegenerating includes: detecting an edge in a previously decoded blockcorresponding to a block adjacent to the current block; andextrapolating or interpolating previously decoded image data along adirection of the edge detected in the detecting to generate thepredicted block, the previously decoded image data corresponding to apixel included in the block adjacent to the current block, and wherein,in the coding, a mode indicator is coded, the mode indicator indicatingthat the edge in the previously decoded block is to be detected togenerate the predicted block.
 35. A non-transitory computer-readablerecording medium on which a program is recorded, the program causing acomputer to execute a method for decoding image data including a codedprediction residual on a block-by-block basis, the method including:decoding the prediction residual of a current block; generating apredicted block by predicting the current block; and adding thepredicted block generated in the generating to the prediction residualdecoded in the decoding to generate a decoded block, wherein the imagedata further includes a mode indicator, and wherein, when the modeindicator indicates that an edge in a previously decoded block is to bedetected to generate the predicted block, the generating includes:detecting the edge in the previously decoded block corresponding to ablock adjacent to the current block; and extrapolating or interpolatingpreviously decoded image data along a direction of the edge detected inthe detecting to generate the predicted block, the previously decodedimage data corresponding to a pixel included in the block adjacent tothe current block.